1. Field of the Invention
The present invention relates to a method of forming a silicon oxide layer on a semiconductor wafer to reduce production of hydrochloric acid (HCl) thus preventing corrosion of the machine used for high temperature oxidation (HTO).
2. Description of the Prior Art
In the semiconductor industry, silicon oxide, with proper dielectric constant and excellent cohesion to silicon surface, is commonly used as a gate oxide, local oxidation of silicon (LOCOS), field oxide, interlayer dielectric, pad oxide, etc. As the number of devices fabricated on the wafer increases, the required quality of the silicon oxide becomes more stringent.
There are three commonly adopted methods of forming a silicon oxide film on the surface of a semiconductor wafer: (1) chemical vapor deposition (CVD), (2) high temperature oxidation (HTO), and (3) spin-on glass (SOG). Generally the silicon oxide film formed by HTO process has the best electrical and physical characteristics. The HTO process according to the prior art includes furnace oxidation and rapid thermal oxidation (RTO) for different furnaces and heating models. In a furnace oxidation process according to the prior art, the silicon oxide layer is formed on the semiconductor wafer by performing a HTO process under the conditions of constant temperature, pressure and volume. Dichlorosilane (DCS, SiH2Cl2) and nitrous oxide (N2O), having a flow rate ratio less than 1:2 respectively, are used as reacting gases. The formula for the main reaction is as follows:
SiH2Cl2+2N2Oxe2x86x92SiO2+2N2+2HClxe2x80x83xe2x80x83(1) 
In daily operations, flow rates of SiH2Cl2 and N2O are often set as 165 standard cubic centimeters per minute (sccm) and 300 sccm respectively, while the inside pressure of the furnace is balanced at 0.45 torr by nitrogen (N2).
Also, a minor reaction simultaneously occurs as SiH2Cl2 is heated. The formula for this minor reaction is as follows:
SiH2Cl2xe2x86x92SiHCl+HClxe2x80x83xe2x80x83(2) 
According to the Ideal Gas Law, the number of moles of the gas is proportional to its volume under the condition of constant temperature and pressure. So the hydrochloric acid, product in both of the main and the minor reactions, is produced at a rate of 315 sccm.
The HCl produced in both the main and minor reaction is highly corrosive to the HTO machine. Besides, as specifications for the semiconductor devices become more and more complicated, the requirement for both the uniformity and deposition rate of the silicon oxide layer tend to be more and more rigid as well. Thus it is important to improve the method of forming silicon oxide layer so as to reduce production of HCl as well as to increase both the deposition rate and uniformity of the silicon oxide layer.
It is therefore a primary object of the present invention to provide a method of forming a silicon oxide layer on a semiconductor wafer so as to prevent corrosion of the high temperature oxidation (HTO) machine due to the hydrochloric acid (HCl) produced.
In the preferred embodiment of the present invention, a silicon oxide layer is formed on a semiconductor wafer by performing a HTO process using dichlorosilane (DCS, SiH2Cl2) and nitrous oxide (N2O) as reacting gases, having a flow rate ratio greater than 2:1 respectively. The reacting moles of SiH2Cl2 to N2O are in the proportion of 1:2 respectively.
It is an advantage of the present invention against the prior art that a DCS-rich surface is firstly formed on the surface of the semiconductor wafer via physical method. So the silicon oxide produced in the subsequent reaction, using SiH2Cl2 and N2O as reacting gases, on the DCS-rich surface can easily adhere to the surface of the semiconductor wafer. The production of HCl is thus reduced and the deposition rate and uniformity of the silicon oxide increases. Therefore, the quality of the semiconductor products can be significantly improved and the corrosion of the HTO machine can be prevented.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the multiple figures and drawings.